Multiple studies have documented significantly lower plasma HIV-1 RNA concentrations (VL) in women compared to men at CD4+ T cell counts greater than 300 cells/mm3. Mechanisms underlying sex differences in VL are unknown. Because most HIV-1 replication occurs in lymphoid tissues (LT), it is likely that differences in LT of men and women account for differences in VL. We propose to explore two distinct theories - that sex differences in CCR5 density on LT CD4+ T cells and/or sex differences in numbers of virions produced per infected LT cell exist - to explain lower plasma VL in women compared to men. The specific aims of this study are: 1) To determine whether CCR5 density on CD4+ T cells in LT is lower in women compared to men;and 2) To determine whether the average number of HIV-1 RNA copies per productively infected lymph node cell is lower in women compared to men. To accomplish this, 30 women and 35 men with HIV-1 infection who are not receiving antiretroviral therapy and who have >300 CD4+ T cells will be recruited to donate inguinal lymph nodes (LN) and peripheral blood. CCR5 density on disaggregated LN CD4+ T cells will be determined by flow cytometric analysis using the QuantiBRITE bead system. RT PCR for HIV-1 RNA will be used to determine VL in plasma and LN cell populations. Frequencies of virus-producing cells in LN sections will be determined by in situ hybridization for HIV-1 RNA and used in conjunction with the LN cell VL to calculate the average number of copies of HIV-1 RNA per infected cell. Data will be analyzed by linear regression, after adjusting for CD4+ T cell count, age, and race, to estimate the relationship between sex, VL (in plasma and LN cells), and density of CCR5 expression or frequencies of virus-producing cells in LT. Average number of copies of HIV-1 RNA in productively infected LN cells will be compared between men and women. Secondary analyses will explore relationships between T cell activation, apoptosis, and sex hormone levels and VL, CCR5 density, and average HIV-1 RNA copy number per cell. A better understanding of mechanisms underlying sex differences in VL could lead to different therapeutic approaches in men and women, different antiretroviral guidelines for HIV-1-infected individuals undergoing hormonal therapies, and novel therapies that improve the treatment of both HIV-1 infected men and women.